In the manufacture of a cast iron engine V-block, a so-called integral barrel crankcase core has been used and consists of a plurality of barrels formed integrally in a crankcase region of the core. The barrels form the cylinder bores in the cast iron engine block without the need for bore liners.
In a sand casting process of an aluminum internal combustion engine cylinder V-block, an expendable mold package is assembled from a plurality of resin-bonded sand cores (also known as mold segments) that define the internal and external surfaces of the engine V-block. Typically, each of the sand cores is formed by blowing resin-coated foundry sand into a core box and curing the sand therein. Cast-in-place bore liners are often used in such castings.
Traditionally, in the manufacture of an aluminum engine V-block with cast-in-place bore liners, the mold assembly method involves positioning a base core on a suitable surface and building up or stacking separate mold elements to shape such casting features as the sides, ends, valley, water jacket, cam openings, and crankcase. The bore liners are positioned on barrel cores such that the liners become embedded in the casting after the metal is poured into the mold. Additional cores may be present as well depending on the engine design. Various designs for the barrel cores are used in the industry. These include individual barrel cores, “V” pairs of barrel cores, barrel-slab cores, and integral barrel crankcase cores. The barrel-slab and integral barrel crankcase designs are often preferred because they provide more accurate positioning of the liners within the mold assembly.
Cast iron bore liners for use in aluminum cylinder block castings are typically machined from centrifugally cast tube stock at a location remote from a foundry, where the engine block is cast. Care must be taken to prevent the machined bore liners from corroding during storage or transportation to the foundry.
The bore liners are typically machined using a cutting fluid. The cutting fluid must be removed from the bore liner prior to the casting operation. Thus, the bore liners are washed to remove the cutting fluid, and any remaining machining chips. A corrosion inhibiting coating is then applied to the bore liners. Special packaging is often used to reduce the risk of corrosion and to protect against other forms of contamination. Corrosion, residual cutting fluid, corrosion inhibiting chemicals, and skin oils associated with fingerprints all cause the formation of a detrimental gas if present on the bore liners when the molten metal is poured. This gas formation is especially evident in the practice of so-called low-pressure foundry processes such as precision sand and semi-permanent mold (SPM) processes.
One known bore liner preconditioning method for low pressure foundry processes typically includes abrasively cleaning the liners using shot blasting, for example. Abrasively cleaning the bore liners removes undesirable contaminants from the bore liner surfaces. Surface roughening caused by the abrasive cleaning also promotes intimate mechanical contact between the aluminum casting and the bore liner (layup). However, abrasive cleaning alone is not sufficient to remove some types of gas producing surface contamination, despite the clean appearance of the bore liner. The bore liners are then assembled into the mold.
A final step of heating the liners to a temperature in the range of 400 to 900 degrees Fahrenheit using an induction heating method is accomplished immediately prior to filling the mold. Heating of the bore liners immediately prior to filling the mold minimizes temperature loss in the molten aluminum as it passes over the bore liners during casting. The heating step also minimizes the risk of cold metal type casting defects and promotes good layup. Special precautions must be taken to avoid bore liner contamination between the abrasive cleaning and heating steps. Additionally, the allowable time delay between heating the bore liners to filling the mold is typically limited to less than four minutes. Delay beyond this time limit results in high risk of cold metal type casting defects.
Another known preconditioning method involves an application of a thin layer of soot or carbon black to the surface of the bore liners. This method does not require a bore liner heating step. Application of the soot layer can minimize the amount scrap caused by gas formation type defects by promoting intimate contact between the aluminum casting and the bore liner. However, the metal pouring temperature must be raised substantially to avoid cold metal type casting defects. Increased metal temperatures can cause undesirable results. The use of soot on non-heated liners also requires special design considerations for gating and other process parameters.
It would be desirable to develop a method for preparing cylinder bore liners wherein an efficiency of the preparation process is maximized, a metal pour temperature is minimized, and material properties of the casting are optimized.